Annular or ring shaped fuel cell unit

ABSTRACT

A fuel cell unit includes a plurality of angularly spaced fuel cell stacks arranged to form a ring-shaped structure about a central axis, each of the fuel cell stacks having a stacking direction extending parallel to the central axis. The fuel cell unit also includes an annular cathode feed manifold surrounding the fuel cell stacks to deliver a cathode feed flow thereto, a plurality of baffles extending parallel to the central axis, each of the baffles located between an adjacent pair of the fuel cell stacks to direct a cathode feed flow from the annular cathode feed manifold and radially inwardly through the adjacent pair, and an annular cathode exhaust manifold surrounded by the fuel cell stacks to receive a cathode exhaust flow therefrom.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of co-pending U.S.patent application Ser. No. 11/503,699, filed Aug. 14, 2006, the entirecontents of which are hereby incorporated by reference.

BACKGROUND

This invention relates to solid oxide fuel cells and the fuel processingassociated therewith.

The Solid oxide fuel cells (“SOFC's”) and associated fuel processors areknown. SOFC's are solid-state devices which use an oxygen ion conductingceramic electrolyte to produce electrical current by transferring oxygenions from an oxidizing gas stream at the cathode of the fuel cell to areducing gas stream at the anode of the fuel cell. This type of fuelcell is seen as especially promising in the area of distributedstationary power generation. SOFC's require an operating temperaturerange which is the highest of any fuel cell technology, giving itseveral advantages over other types of fuel cells for these types ofapplications. The rate at which a fuel cell's electrochemical reactionsproceed increases with increasing temperature, resulting in loweractivation voltage losses for the SOFC. The SOFC's high operatingtemperature can preclude the need for precious metal catalysts,resulting in substantial material cost reductions. The elevated exittemperature of the flow streams allow for high overall systemefficiencies in combined heat and power applications, which are wellsuited to distributed stationary power generation.

The traditional method of constructing solid oxide fuel cells has beenas a large bundle of individual tubular fuel cells. Systems of severalhundred kilowatts of power have been successfully constructed using thismethodology. However, there are several known disadvantages to thetubular design which severely limit the practicality of its use in thearea of 25 kW-100 kW distributed stationary power generation. Forexample, producing the tubes can require expensive fabrication methods,resulting in achievable costs per kW which are not competitive withcurrently available alternatives. As another example, the electricalinterconnects between tubes can suffer from large ohmic losses,resulting in low volumetric power densities. These disadvantages to thetubular designs have led to the development of planar SOFC designs. Theplanar designs have been demonstrated to be capable of high volumetricpower densities, and their capability of being mass produced usinginexpensive fabrication techniques is promising.

As is known in the art, a single planar solid oxide fuel cell (SOFC)consists of a solid electrolyte which has high oxygen ion conductivity,such as yttria stabilized zirconia (YSZ); a cathode material such asstrontium-doped lanthanum manganite on one side of the electrolyte,which is in contact with an oxidizing flow stream such as air; an anodematerial such as a cermet of nickel and YSZ on the opposing side of theelectrolyte, which is in contact with a fuel flow stream containinghydrogen, carbon monoxide, a gaseous hydrocarbon, or a combinationthereof such as a reformed hydrocarbon fuel; and an electricallyconductive interconnect material on the other sides of the anode andcathode to provide the electrical connection between adjacent cells, andto provide flow paths for the reactant flow streams to contact the anodeand cathode. Such cells can be produced by well-established productionmethodologies such as screen-printing and ceramic tape casting.

However, there are still challenges to implementing the planar SOFC forstationary power generation in the range of 25 kW-100 kW. The practicalsize of such cells is currently limited to a maximum footprint ofapproximately 10×10 cm by issues such as the thermal stresses within theplane of the cell during operation and the difficulties involved infabricating very thin components. Since the achievable power density ofthe fuel cell is in the range of 180-260 mW/cm2, a large number of cellsmust be assembled into one or more fuel cell stacks in order to achievethe required power levels for a stationary power generation application.Implementing large numbers of such cells presents several difficulties.A planar SOFC design requires high-temperature gas-tight seals aroundthe edges of the cells, which typically requires large compressive loadson the stack. Anode and cathode flowstreams must be evenly distributedamong the many cells. The heat generated by the fuel cell reaction mustbe able to be removed from the stack in order to prevent overheating.These issues and others have made it difficult for planar SOFCmanufacturers to progress to fuel cell systems larger than about 5 kWe.

Thus, while the known systems may be suitable for their intendedpurpose, there is always room for improvement.

SUMMARY

In one aspect, the invention provides a fuel cell unit including aplurality of angularly spaced fuel cell stacks arranged to form aring-shaped structure about a central axis, each of the fuel cell stackshaving a stacking direction extending parallel to the central axis,wherein each of the stacks has a rectangular cross section. The fuelcell unit also includes an annular cathode feed manifold surrounding thefuel cell stacks to deliver a cathode feed flow thereto, a plurality ofbaffles extending parallel to the central axis, each of the baffleslocated between an adjacent pair of the fuel cell stacks to direct acathode feed flow from the annular cathode feed manifold and radiallyinwardly through the adjacent pair, and an annular cathode exhaustmanifold surrounded by the fuel cell stacks to receive a cathode exhaustflow therefrom.

In another aspect, the invention provides a fuel cell unit including anannular array of fuel cell stacks surrounding a central axis, with eachof the fuel cell stacks having a stacking direction extending parallelto the central axis, wherein each of the stacks has a rectangular crosssection. The fuel cell unit also includes an annular cathode feedmanifold surrounding the annular array of fuel cell stacks to deliver acathode feed flow thereto, a plurality of baffles extending parallel tothe central axis, each of the baffles located between an adjacent pairof the fuel cell stacks to direct a cathode feed flow from the annularcathode feed manifold and radially inwardly through the adjacent pair,and an annular cathode exhaust manifold surrounded by the annular arrayof fuel cell stacks to receive a cathode exhaust flow therefrom.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a fuel cell unit with an integrated SOFCand fuel processor embodying the present invention.

FIGS. 2A and 2B are sectional views showing one half of the fuel cellunit of FIG. 1, with FIG. 2A illustrating the flows of the cathode feedand exhaust gases and FIG. 2B illustrating the flows of the anode feedand exhaust gases.

FIG. 3A is a sectional view taken from line 3A-3A in FIG. 1, but showingonly selected components of the fuel cell unit.

FIG. 3B is an enlarged, somewhat schematic view taken from line 3B-3B inFIG. 3A.

FIG. 4A is an enlarged, perspective view of a cathode flow side of afuel cell plate/interconnect for use in the unit of FIG. 1.

FIG. 4B is a view similar to FIG. 4A, showing the opposite side of thefuel cell plate/interconnect, which is the anode flow side.

FIG. 5 is an exploded perspective view showing an integrated pressureplate/anode feed manifold and an array of fuel reformer tubes togetherwith other selected components of the integrated unit of FIG. 1.

FIG. 6 is a perspective view showing the components of FIG. 5 in theirassembled state.

FIG. 7 is a partial section view illustrating construction detailscommon to several heat exchangers contained within the integrated unitof FIG. 1.

FIGS. 8 and 9 are exploded perspective views of the components of ananode exhaust cooler of the integrated unit of FIG. 1.

FIG. 10 is a perspective view showing the components of FIGS. 8 and 9 intheir assembled state.

FIG. 11 is an exploded perspective view showing the assembled componentsof FIGS. 6 and 10 together with an anode recuperator of the integratedunit of FIG. 1.

FIG. 12 is an exploded perspective view showing the components of FIG.11 together with a reformer catalyst insert and a cover ring componentof the integrated unit of FIG. 1.

FIG. 13 is an enlarged, exploded perspective view of selected componentsutilized to distribute and collect anode flow to the fuel cell stacks ofthe integrated unit of FIG. 1.

FIG. 14 is a perspective view showing the assembled components of FIG.13.

FIG. 15 is an exploded perspective view showing the assembled unit ofFIG. 12 together with an annular array of fuel cell stacks of theintegrated unit of FIG. 1.

FIGS. 16-19 are views similar to FIG. 14 with each showing additionalcomponents of the array of fuel cell stacks as they are assembled;

FIG. 20 is an exploded perspective view showing the components of FIG.19 in their assembled state together with a plurality of spacer/baffles;

FIG. 21 is an enlarged, broken perspective view showing the componentsof FIG. 20 in their assembled state.

FIG. 22 is an exploded perspective view showing the assembled componentsof FIG. 20 together with an upper pressure plate and a plurality of tierods.

FIG. 23 is a perspective view showing the components of FIG. 22 in theirassembled state.

FIG. 24 is an exploded perspective view showing the components of FIG.23 together with an insulation disk and heat shield housing of theintegrated unit of FIG. 1.

FIG. 25 is a perspective view showing the assembled state of thecomponents of FIG. 24.

FIG. 26 is an exploded perspective view showing a cathode recuperatorassembly together with other components of the integrated unit of FIG.1.

FIG. 27 is an exploded perspective view showing the assembled componentsof FIG. 26 together with the assembled components of FIG. 24.

FIG. 28 is an exploded perspective view showing the assembled componentsof FIG. 27 together with an outer housing of the integrated unit of FIG.1.

FIG. 29 is an enlarged, partial perspective section view showingselected components of the unit of FIG. 1.

FIG. 30 is a view similar to FIG. 1, but showing a modified version ofthe integrated SOFC and fuel processor.

FIG. 31 is an exploded perspective view of a steam generator utilized inthe integrated unit of FIG. 30.

FIG. 32 is a perspective view of the steam generator of FIG. 31.

FIG. 33 is a schematic representation of the fuel cell units embodyingthe invention.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

With reference to FIGS. 1, 2A, 2B and 3A; an integrated fuel cell unit10 is shown in form of an integrated solid oxide fuel cell (“SOFC”)/fuelprocessor 10 having a generally cylindrical construction. The unit 10includes an annular array 12 of eight (8) fuel cell stacks 14surrounding a central axis 16, with each of the fuel cell stacks 14having a stacking direction extended parallel to the central axis 16,with each of the stacks having a face 17 that faces radially outward anda face 18 that faces radially inward. As best seen in FIG. 3A the fuelcell stacks 14 are spaced angularly from each other and arranged to forma ring-shaped structure about the axis 16. Because there are eight ofthe fuel cell stacks 14, the annular array 12 could also becharacterized as forming an octagon-shaped structure about the axis 16.While eight of the fuel cell stacks 14 have been shown, it should beunderstood that the invention contemplates an annular array 12 that mayinclude more than or less than eight fuel cell stacks.

With reference to FIG. 1, the unit 10 further includes an annularcathode recuperator 20 located radially outboard from the array 12 offuel stacks 14, an annular anode recuperator 22 located radially inboardfrom the annular array 12, a reformer 24 also located radially inboardof the annular array 12, and an annular anode exhaust cooler/cathodepreheater 26, all integrated within a single housing structure 28. Thehousing structure 28 includes an anode feed port 30, an anode exhaustport 32, a cathode feed port 34, a cathode exhaust port 36, and an anodecombustion gas inlet port 37. An anode exhaust combustor (typically inthe form an anode tail gas oxidizer (ATO) combustor), shownschematically at 38, is a component separate from the integrated unit 10and receives an anode exhaust flow 39 from the port 32 to produce ananode combustion gas flow 40 that is delivered to the anode combustiongas inlet 37. During startup, the combustor 38 also receives a fuel flow(typically natural gas), shown schematically by arrow 41. Additionally,some of the anode exhaust flow may be recycled to the anode feed port30, as shown by arrows 42. In this regard, a suitable valve 43 may beprovided to selectively control the routing of the anode exhaust flow toeither the combustor 38 or the anode feed port 30. Furthermore, althoughnot shown, a blower may be required in order to provide adequatepressurization of the recycled anode exhaust flow 42. While FIGS. 1, 2Aand 2B are section views, it will be seen in the later figures that thecomponents and features of the integrated unit 10 are symmetrical aboutthe axis 16, with the exception of the ports 34, 36 and 37.

With reference to FIG. 1 and FIG. 2A, the cathode flows will beexplained in greater detail. As seen in FIG. 1, a cathode feed(typically air), shown schematically by arrows 44, enters the unit 10via the port 34 and passes through an annular passage 46 before enteringa radial passage 48.

It should be noted that as used herein, the term “radial passage” isintended to refer to a passage wherein a flow is directed eitherradially inward or radially outward in a generally symmetric 360°pattern. The cathode feed 44 flows radially outward through the passage48 to an annular passage 50 that surrounds the array 12 and passesthrough the cathode recuperator 20. The cathode feed 44 flows downwardthrough the annular passage 50 and then flows radially inward to anannular feed manifold volume 52 that surrounds the annular array 12 todistribute the cathode feed 44 into each of the fuel cell stacks 14where the cathode feed provides oxygen ions for the reaction in the fuelcell stacks 14 and exits the fuel cell stacks 14 as a cathode exhaust56. The cathode exhaust 56 then flows across the reformer 24 into anannular exhaust manifold area 58 where it mixes with the combustion gasflow 40 which is directed into the manifold 58 via an annular passage60. In this regard, it should be noted that the combustion gas flow 40helps to make up for the loss of mass in the cathode exhaust flow 56resulting from the transport of oxygen in the fuel cell stacks 14. Thisadditional mass flow provided by the combustion gas flow 40 helps inminimizing the size of the cathode recuperator 20. The combinedcombustion gas flow 40 and cathode exhaust 56, shown schematically byarrows 62, exits the manifold 58 via a central opening 64 to a radialpassage 66. The combined exhaust 62 flows radially outward through thepassage 66 to an annular exhaust flow passage 68 that passes through thecathode recuperator 20 in heat exchange relation with the passage 50 totransfer heat from the combined exhaust 62 to the cathode feed 44. Thecombined exhaust 62 flows upward through the annular passage 68 to aradial passage 70 which directs the combined exhaust 62 radially inwardto a final annular passage 72 before exiting the unit 10 via the exhaustport 36.

With reference to FIG. 1 and FIG. 2B, an anode feed, shown schematicallyby arrows 80, enters the unit 10 via the anode feed inlet port 30preferably in the form of a mixture of recycled anode exhaust 42 andmethane. The anode feed 80 is directed to an annular passage 82 thatpasses through the anode recuperator 22. The anode feed 80 then flows toa radial flow passage 84 where anode feed 80 flows radially outward toan annular manifold or plenum 86 that directs the anode feed into thereformer 24. After being reformed in the reformer 24, the anode feed 80exits the bottom of reformer 24 as a reformate and is directed into anintegrated pressure plate/anode feed manifold 90. The feed manifold 90directs the anode feed 80 to a plurality of stack feed ports 92, withone of the ports 92 being associated with each of the fuel cell stacks14. Each of the ports 92 directs the anode feed 80 into a correspondinganode feed/return assembly 94 that directs the anode feed 82 into thecorresponding fuel cell stack 14 and collects an anode exhaust, shownschematically by arrows 96, from the corresponding stack 14 after theanode feed reacts in the stack 14. Each of the anode feed/returnassemblies 94 directs the anode exhaust 96 back into a corresponding oneof a plurality of stack ports 98 in the pressure plate/manifold 90(again, one port 98 for each of the fuel cell stacks 14). The manifold90 directs the anode exhaust 96 radially inward to eight anode exhaustports 100 (again, one for each stack 14) that are formed in the pressureplate/manifold 90. The anode exhaust 96 flows through the ports 100 intoa plurality of corresponding anode exhaust tubes 102 which direct theanode exhaust 96 to a radial anode exhaust flow passage 104. The anodeexhaust 96 flows radially inward through the passage 104 to an annularflow passage 106 that passes downward through the anode recuperator 22in heat exchange relation with the flow passage 82. The anode exhaust 96is then directed from the annular passage 106 upward into a tubularpassage 108 by a baffle/cover 110 which is preferably dome-shaped. Theanode exhaust 96 flows upwards through the passage 108 before beingdirected into another annular passage 112 by a baffle/cover 114, whichagain is preferably dome-shaped. The annular passage 112 passes throughthe anode cooler 26 in heat exchange relation with the annular cathodefeed passage 46. After transferring heat to the cathode feed 44, theanode exhaust 96 exits the annular passage 112 and is directed by abaffle 116, which is preferably cone-shaped, into the anode exhaust port32.

Having described the primary components of the unit 10 and the flowpaths for the cathode and anode flows, the details of each of thecomponents will now be discussed. In this regard, while the discussionwill often refer to the figures out of numerical order, the numericalorder of most of the figures was selected to reflect the assemblysequence of the unit 10.

Turning now in greater detail to the construction of the array 12 offuel cell stacks 14, as best seen in FIGS. 1 and 15-19 in theillustrated embodiment, each stack 14 includes four substacks 120 witheach of the substacks 120 including multiple individual planar SOFCcells 122, shown schematically in FIGS. 1 and 15-19, which are stackedso that they are in electrical series. The number of cells required foreach substack 120 will be dependent upon the ability to distribute theanode flow with enough uniformity for satisfactory performance but maytypically be between fifty (50) and one hundred (100) cells 122. Foreach of the cells 122, the structure of the electrolyte, anode, cathode,interconnects, and seal can be fabricated by any suitable method, manyof which are known in the art of planar solid oxide fuel cells. Asexamples, the cell components can be electrolyte supported or anodesupported, they can be fabricated by ceramic tape casting or otherwell-known means of construction, and the seals between the cells can bethe glass ceramic or metallic type. In the illustrated embodiment, theanode sides of the cells 122 are internally manifolded within eachsubstack 120, while the cathode sides are externally manifolded via themanifolds 52 and 58 as previously discussed. FIGS. 4A and 48 showpossible designs for a flow plate/interconnect 124, with FIG. 4A showingcathode flow paths on one side and FIG. 48 showing the anode flow pathson the opposite side to direct the cathode and anode flow streams in acounter-flow manner. It can be seen that the cathode side includes aplurality of parallel, linear flow paths 128 that are open to eitherface 17,18 of the fuel cell 122 to allow passage of the cathode feed 44through the fuel cell 122. The plate 124 also includes openings 130 and132 that are surrounded by bosses on the cathode side which can besealed in order to form the internal manifolding for the anode feed andexhaust flows 80 and 96. On the anode side, the openings 130 and 132allow for entry and exit of the anode feed and exhaust flows 80 and 96,respectively, with the opening 130 feeding a linear plenum 138 thatdirects the anode feed flow to a plurality of parallel, linear flowpaths 140, and a linear plenum 142 that directs the anode exhaust flow96 to the opening 132. A single solid oxide fuel cell consisting of acathode layer, a ceramic electrolyte layer, and an anode layer issandwiched between each adjacent pair of the cathode flow paths 128 andthe anode flow paths 140 in each of the stacks 14, and an electriccurrent is produced by transferring oxygen ions from the cathode flow 44through the ceramic electrolyte layer to the anode feed flow 80according to the following reactions:Cathode: O²+4e ⁻→2O² ⁻Anode: H₂+O² ⁻ →H₂O+2e ⁻CO+O² ⁻ →CO₂+2e ⁻

With reference to FIGS. 13 and 14, each of the feed/return assemblies 94includes an anode feed tube 160, an anode exhaust tube 162, a pair ofcover plates 164 and 166, an intermediate plate 168, and a pair of fluidconnections 170 and 172. In the illustrated embodiment, the plates 164and 166 are identical and each plate 164 and 166 includes a feed port174, an exhaust port 176, a feed opening 178, an exhaust opening 180,and a clearance hole 182. The intermediate plate 168 includes aclearance hole 184, a feed slot 186 and an exhaust slot 188. In theassembled state, the plates 164-168 form a splitter manifold 189 and thefeed slot 186 directs the anode feed 80 from the ports 174 to theopenings 178 for delivery to the substacks 120 positioned above andbelow the manifold 189, while the exhaust slot 188 directs anode exhaust96 from the exhaust openings 180 to the ports 176 after receiving theanode exhaust from the substacks 120 positioned above and below themanifold 89. The fluid connections 170 and 172 either serve to connectthe manifold assembly 94 to the tubes 160 and 162 of the next anodesplitter assembly or, for the topmost splitter assembly, are provided inthe form of end caps that close the ports 174 and 176. As will beexplained in more detail below, the clearance holes 182 and 184 provideclearance for a bolt that is used to maintain alignment between thesubstacks 120 adjacent the splitter manifold 189.

With reference to FIGS. 1 and 20, it can be seen that for each stack 14,the lowermost assemblies 94 service the two lower substacks 120, whilethe uppermost assemblies 94 service the two upper substacks 120.

With reference to FIGS. 13 and 14, preferably, each of the tubes 160 and162 includes a pair of metallic tubes/bellows 190 to accommodate thermalexpansion of the corresponding stack 14. Each pair of tubes/bellows 190is connected by a tube-shaped electrical isolator 192 made of a suitablematerial that can be bonded (such as by brazing or by epoxy) to thetubes/bellows 190. The electrical isolators 192 provide electricalisolation of the manifold 189 from the manifold 90 and other manifolds189.

With reference to FIGS. 15-19, it can be seen that the lowermostsubstack 120, the combination of the two intermediate substacks 120, andthe uppermost substack 120 are each sandwiched between a pair of currentcollector plates 200, with each of the plates 200 including a tab 202having a bolt opening 203 therein, an anode feed opening 204 that alignswith the corresponding feed opening 178 in the corresponding manifold189 for transferring the anode feed 82 from the manifold 189 and to thecorresponding substack 120, and an anode exhaust opening 206 that alignswith the corresponding exhaust opening 180 of the corresponding manifold89 to direct the anode exhaust 96 from the corresponding substack 120into the manifold 89. As best seen in FIGS. 13 and 17-19, bolts 208 areused to align and sandwich the current collector plates 200 on eitherside of a corresponding assembly 94 by passing the bolt 208 through theopenings 182, 184 and 203 of the corresponding plates 164, 166, 168 and200 and clamping the bolt with a corresponding washer and nut (notshown).

As best seen in FIG. 23, bolt-like threaded electrodes 210 are providedthrough the openings 203 of the lowermost and uppermost collector plates200 in order to provide bus connections for each of the stacks 14, withthe upper electrodes 210 being surrounded by a can-shaped electrodesleeve 211 that shields the upper electrodes 210 from the cathode feed44 and combined exhaust 62 in the passages 48 and 70. As will beexplained in more detail below, the sleeve 211 also provides a sealsurface for retaining the various flows of the unit 10 and allows forthe electrode 210 to be electrically isolated from the various housingcomponents of the unit 10. As best seen in FIG. 15, a layer ofelectrical insulation 212 is sandwiched between each of the lowermostcollector plates 200 and the pressure plate/manifold 90 to electricallyisolate the manifold 90 from the stacks 14.

With references to FIGS. 20-22, it can be seen that wedge-shapedspacers/flow baffles 220 are provided between adjacent pairs of thestacks 14. The baffles 220 serve to direct the cathode feed 44 into thecathode flow paths 128 and to fill the space between adjacent stacks sothat the cathode feed 44 passes through each of the stacks 14, ratherthan bypassing around the longitudinal sides of the stacks 14. As seenin FIG. 22, the baffles 220 are held in place by tie rods 222 that passthrough closely fitting bores 224 centrally located in each of thebaffles 220. Preferably, the baffles 220 are electrically non-conductiveand made as one unitary piece from a suitable ceramic material. While aunitary construction is preferred for the baffles 220, it may bedesirable in some applications to provide the baffles as a multi-piececonstruction wherein only those parts of the baffle that contact thestacks 14 need to be electrically non-conductive. As best seen in FIG.21, while optional, it is preferred that each of the baffles 220includes a pair of longitudinal lips or wings 226 that extend slightlyover the radially outer face 17 of the stacks 14 in order to furtherrestrict the bypassing of the cathode feed 44 around the longitudinalsides of the stacks 14. In this regard, it should be appreciated thatthermal growth in the circumferential direction will tend to decreasethe sealing effect of the baffles 220 against the longitudinal sides ofthe stacks 14 because of the greater thermal growth of the metallicpressure plates between which the stacks 14 are sandwiched in comparisonto the thermal growth of the stacks and baffles in the circumferentialdirection. The wings 226 help to prevent bypassing of the cathode flowthat could otherwise be the result of such thermal growth.

With reference to FIG. 22, the stacks 14 are compressed between theintegrated pressure plate/manifold 90 and an upper pressure plate 230 bypassing the rods 222 through the pressure plate 230 and engaging thebottom side of the pressure plate 90 via a compression spring assembly231 including an upper and lower pair of washers 232 that sandwich acompression spring (or a stack of die springs) 234 and are loaded by athreaded nut 236 engaging the threads on the end of the tie rod 220 toprovide the compression force through the stacks 14. The compressionspring assemblies 231 allow for thermal growth differential of themetallic tie rods 220 with respect to the largely ceramic stacks 14during operation. The compression also helps to minimize the areaspecific electrical resistance in each of the stacks 14, and helps tomaintain the seals that are formed between the interfacing plates of thestacks 14 for the cathode and anode gas flows. It should be noted thatthe illustrated embodiment of the unit 10 also includes a boltflange/mount plate assembly 237 between the spring assemblies 231 andthe pressure plate 90 to provide interfacing structure 238 for asupporting base 239 of the unit 10 and serve as the bottom cover for thehousing 28 of the unit 10. The assembly 237 is spaced off of thepressure plate 90 to form the exhaust flow passage 66. Although notshown, electrical insulating layers of a suitable material are locatedbetween the pressure plate 230 and the stacks 12 in order toelectrically isolate the stacks 12 from the pressure plates 90 and 230and the rest of the compression components.

Referring back to FIG. 5, it can be seen that the pressureplate/manifold assembly 90 includes a pair of cover plates 240 and 242that sandwich a plurality of intermediate plates 244, 246, 248 and 250.The plates 240, 242, 244, 246 and 248 all include eight equally spaced,tie rod through holes 252 that align with the holes 252 in the otherplates to allow passage of the tie rods 222 through the manifold 90. Theplates 242, 244, 246 and 248 each also include sixteen equally spacedsomewhat triangular-shaped tabs 253 extending from their peripheries andin alignment with the corresponding tabs 253 on the other plates.Additionally, the plate 240 includes eight equally spaced openings 254that allow the electrodes 210 to pass through the plate 240. The uppercover plate 242 includes the ports 92 and 98 for the anode feed andexhaust respectively, as well as the eight ports 100 for the directingthe anode exhaust 96 to the eight tubes 102. Fluid connectors 255similar to the connectors 170,172 are provided for each of the ports 92and 98. The intermediate plate 244 includes eight anode exhaust slots256 for directing the anode exhaust 96 from the eight ports 98 to theeight ports 100. Eight openings 260 and 262 are provided in the˜plates246 and 248, respectively, and are aligned with the eight ports 98 andone end of the eight slots 256 in order to direct the anode exhaust 96from the port 98 into the slot 256. Eight openings 264 and 266 areprovided in the plates 246 and 250, respectively, and are aligned withan opposite end of the slots 256 in the plate 244 and with the ports 100in the plate 242 in order to direct the anode exhaust 96 from the slots256 into the ports 100. The plate 248 includes eight radially directedanode feed slots 270 that are connected into a central opening 272 ofthe plate 248 that forms an annular plenum 274 with an outer perimeterof the plate 250. The eight ports 92 of the plate 242 are aligned withone end of the eight slots 270 in order to receive the anode feed 80therefrom. Eight sets of reformer tube receiving slots 276 (only twosets of the slots 270 are shown in FIG. 5) are provided in the plate 242so as to overlie the annular plenum 274 formed between the plates 248and 250 in order to direct the anode feed 80 from the reformer 24 intothe annular plenum 274 for supplying the slots 270. Aligned centralopenings 278 having conforming inner perimeters are provided in theplates 240, 242, 244, 246 and 250 in order to allow passage of othercomponents of the unit 10 through the assembly 90 and to define thecentral opening 64 previously described in connection with the flow ofthe cathode exhaust 62. It should be appreciated that the features ofintermediate plates 244, 246, 248 and 250 could alternatively beprovided in a single machined plate of thickness equal to the totalthickness of plates 246, 248 and 250.

With reference to FIGS. 3A, 3B and to FIG. 5, the reformer 24 isprovided in the form of an annular array 280 of eight tube sets 282,with each tube set 282 corresponding to one of the fuel cell stacks 14and including a row of flattened tubes 284. In this regard, it should benoted that the number of tubes 284 in the tube sets 282 will be highlydependent upon the particular parameters of each application and canvary from unit 10 to unit 10 depending upon those particular parameters.Thus, FIGS. 3A and 3B illustrate five of the tubes 284 for each of thetube sets 282, whereas FIG. 5 illustrates ten of the tubes 284 for eachof the tube sets 282.

Preferably, the reformer is a steam methane reformer (“SMR”). Steammethane reforming is a well-known process is which methane (i.e. naturalgas) is reacted with steam over a catalyst to produce hydrogen. Thesteam reforming process consists of two separate reactions which occurwithin the same reactor—an oxygenolysis reaction (typically referred toas the steam reforming reaction) and an associated water-gas shiftreaction. The oxygenolysis reaction produces hydrogen and carbonmonoxide as follows:CH₄+H₂O→3H₂+CO

This reaction is highly endothermic, requiring 206 kJ of energy per moleof methane consumed. Some of the CO produced is converted to CO 2 viathe associated water-gas shift reaction:CO+H₂O→C0₂+H₂

This reaction is moderately exothermic, and liberates 41 kJ of energyper mole of CO consumed. Steam reforming of methane for fuel cells istypically carried out over a precious metal catalyst at temperatures inthe range of 700° C.-900° C. Since the overall reaction is endothermic,heat must be supplied to the reactor. It is advantageous from a systemefficiency standpoint to utilize the heat produced by the solid oxidefuel cells 222 as the heat source for the reformer.

The steam methane reforming takes place as the anode feed 80 passesthrough the interior of the tubes 284 and comes in contact with asuitable catalyst (typically a precious metal catalyst) contained withinthe tubes 284. In this regard, as best seen in FIGS. 3B and 12, catalystcoated inserts 286, such as serpentine fins or lanced and offset fins,can be placed inside each of the tubes 284 to increase the catalystsurface area for the anode feed 80. While the inserts 286 can be brazedinside of the tubes 284, in the illustrated embodiment the inserts 286are placed into the tubes 284 after brazing, as shown in FIG. 11. Inthis regard, although not shown, an insert support ring can be placedwithin the annular plenum 274 of the manifold assembly 90 if required tosupport the particular structure of the insert 286.

As best seen in FIGS. 3A and 3B, the tubes 284 in each of the sets 282are preferably arranged relative to the exit face 18 of thecorresponding fuel cell stack 14 to ensure that the majority of theradiant heat energy from the fuel cell stack 14 cannot pass through thetube set 282 without impinging on one of the broad sides of the tubes284. To this end, the tubes 284 in each set 282 are arranged relative tothe corresponding fuel cell stack 14 to ensure that radiant heat energyradiating normal to the face 18 cannot pass through the tube set 282without impinging on one of the broad sides of the tubes 284, as bestseen in FIG. 3B. To state this in other terms, the tubes 284 arearranged so that there is no direct “line-of-sight” normal to the face18 through the tube set 282 from the perspective of the face 18 of thecorresponding fuel cell stack 14. It should be appreciated that theparticular angle α selected for the tubes 284 in each tube set 282 willdepend upon the tube-to-tube spacing as well as the major dimension ofeach of the tubes 284. This arrangement of the tubes 284 helps tomaximize the heating of the reformer 24, which is also heated by thecathode exhaust 56 as it passes over the exterior of the tubes 284. Itshould also be noted that the tubes 284 of the reformer also receiveradiant heat energy from the cylindrical wall 290 that defines the flowpassage 60 for the anode combustion gas 40 that flows into the manifoldarea 58. In this regard, it should be appreciated that the tubes arealso arranged relative to the wall 290 to ensure that radiant heatenergy radiating normal to the surface of the wall 290 at any pointcannot pass through the corresponding set of tubes 282 without impingingon one of the broad sides of the tubes 282.

A plenum or manifold plate 292 is provided to distribute the anode feed80 to the interiors of the tubes 284 and includes a plurality of tubereceiving slots 294 having an arrangement (like that of the slots 276)that corresponds to the ends of the tubes 284 in the array 280 so as toreceive the ends of the tubes 284 in a sealed relation when brazed orotherwise bonded to the tubes 284 (again as with the slots 276). Themanifold plate 292 also includes eight equally spaced, through holes 296which receive ends of the eight anode exhaust tubes 102 and aresealed/bonded thereto. A central opening 298 is provided in the plate292 to receive other components of the unit 10. As shown in FIG. 6, theabove-described components of the pressure plate/manifold assembly 90and the reformer 24 preferably are assembled and brazed as a singlesubassembly.

FIG. 7 is intended as a generic figure to illustrate certainconstruction details common to the cathode recuperator 20, the anoderecuperator 22, and the anode cooler 26. The construction of each ofthese three heat exchangers basically consists of three concentriccylindrical walls A,B,C that define two separate flow passages D and E,with corrugated or serpentine fin structures G and H provided in theflow passages D and E, respectively, to provide surface areaaugmentation of the respective flow passages. Because the heat transferoccurs through the cylindrical wall B, it is preferred that the fins Gand H be bonded to the wall B in order to provide good thermalconductivity, such as by brazing. On the other hand, for purposes ofassembly and/or allowing differential thermal expansion, it is preferredthat the fins G and H not be bonded to the cylindrical walls A and C.For each of the heat exchangers 20, 22 and 26, it should be understoodthat the longitudinal length and the specific geometry of the fins G andH in each of the flow paths D and E can be adjusted as required for eachparticular application in order to achieve the desired outputtemperatures and allowable pressure drops from the heat exchangers.

Turning now to FIGS. 8-10, the anode cooler 26 includes a corrugated orserpentine fin structure 300 to provide surface area augmentation forthe anode exhaust 96 in the passage 112, a corrugated or serpentine finstructure 302 that provides surface area augmentation for the cathodefeed flow 44 in the passage 46, and a cylindrical wall or tube 304 towhich the fins 300 and 302 are bonded, preferably by brazing, and whichserves to separate the flow passage 46 from the flow passage 112. Asbest seen in FIG. 9, a cylindrical flow baffle 306 is provided on theinterior side of the corrugated fin 300 and includes the dome-shapedbaffle 114 on its end in order to define the inner part of flow passage112. A donut-shaped flow baffle 308 is also provided to direct thecathode feed 44 radially outward after it exists the flow passage 46.The cone-shaped baffle 116 together with the port 32 are attached to thetop of the tube 304, and include a bolt flange 310 that is structurallyfixed, by a suitable bonding method such as brazing or welding, to theport 32, which also includes a bellows 311 to allow for thermalexpansion between the housing 28 and the components connected throughthe flange 310. As seen in FIG. 10, the above-described components canbe assembled as yet another subassembly that is bonded together, such asby brazing.

In reference to FIGS. 1 and 11, it can be seen that the anoderecuperator 22 includes a corrugated or serpentine fin structure 312(best seen in FIG. 8) in the annular flow passage 82 for surface areaaugmentation for anode feed 80. As best seen in FIG. 1, the anoderecuperator 22 further includes another corrugated or serpentine finstructure 314 in the annular flow passage 106 for surface augmentationof the anode exhaust 96. As best seen in FIG. 11, corrugated fins 312and 314 are preferably bonded to a cylindrical wall of tube 316 thatserves to separate the flow passages 82 and 106 from each other, withthe dome-shaped baffle 110 being connected to the bottom end of the wall316. Another cylindrical wall or tube 320 is provided radially inboardfrom the corrugated fin 314 (not shown in FIG. 11, but in a locationequivalent to fin 300 in cylinder 304 as seen in FIG. 9) to define theinner side of the annular passage 106, as best seen in FIG. 11. As seenin FIG. 2A, an insulation sleeve 322 is provided within the cylindricalwall 320 and a cylindrical exhaust tube 324 is provided within theinsulation sleeve 322 to define the passage 108 for the anode exhaust96. Preferably, the exhaust tube 324 is joined to a conical-shapedflange 328 provided at a lower end of the cylindrical wall 320. Withreference to FIG. 11, another cylindrical wall or tube 330 surrounds thecorrugated fin 312 to define the radial outer limit of the flow passage82 and is connected to the inlet port 30 by a conical-shaped baffle 332.A manifold disk 334 is provided at the upper end of the wall 316 andincludes a central opening 336 for receiving the cylindrical wall 320,and eight anode exhaust tube receiving holes 338 for sealingly receivingthe ends of the anode exhaust tubes 102, with the plate 308 serving toclose the upper extent of the manifold plate 334 in the assembled state.As seen in FIG. 12, the previously described components of the anodecooler 26 and the anode recuperator 22 are inserted through a centralopening 298 of the manifold plate 292 with the ends of the tubes 102being received and sealingly bonded in the openings 338 and the top ofthe cylindrical wall 330 being sealingly bonded to the perimeter of theopening 298 to define the flow path for the anode feed 80 into theradial passage 84. A ring-shaped manifold plate 340 with flanges 342 and344 at its inner and outer perimeter is provided to enclose the areadefined by the manifold plate 292 and the plate 334 so as to define themanifold 86 for distributing the anode feed flow from the radial passage84 to the interior of the tubes 284.

With reference to FIGS. 28 and 24, a heat shield assembly 350 is shownand includes an inner cylindrical shell 352 (shown in FIG. 28), an outercylindrical shell 354, an insulation sleeve 356 (shown in FIG. 28)positioned between the inner and outer shells 352 and 354, and adisk-shaped cover 358 closing an open end of the outer shell 350. Thecover 358 includes eight electrode clearance openings 360 for throughpassage of the electrode sleeves 211. As seen in FIG. 24, the heatshield assembly 350 is assembled over an insulation disk 361 the outerperimeter of the assembled array 12 of fuel cells 14 and defines theouter extent of the cathode feed manifold 52. The heat shield 350 servesto retain the heat associated with the components that it surrounds.

With reference to FIG. 1 and FIG. 26, the cathode recuperator 20includes a corrugated or serpentine fin structure 362 to provide surfaceenhancement in the annular flow passage 68 for the combined exhaust 62,a corrugated or serpentine fin structure 364 to provide surfaceenhancement in the annular flow passage 50 for the cathode feed 44, anda cylindrical tube or wall 366 that separates the flow passages 50 and68 and to which the fins 362 and 364 are bonded. A disk-shaped coverplate 368 is provided to close the upper opening of the cylindrical wall366 and includes a central opening 370, and a plurality of electrodeclearance openings 372 for the passage of the electrode sleeve 211therethrough. A cylindrical tube or sleeve 376 is attached to the cover368 to act as an outer sleeve for the anode cooler 26, and an upperannular bolt flange 378 is attached to the top of the sleeve 376. Alower ring-shaped bolt flange 380 and an insulation sleeve 382 arefitted to the exterior of the sleeve 376, and a cylindrical wall orshield 384 surrounds the insulation sleeve 382 and defines an inner wallfor the passage 72, as best seen in FIGS. 1 and 26.

With reference to FIG. 27, the components of FIG. 26 are then assembledover the components shown in FIG. 25 with the flange 378 being bolted tothe flange 310.

With reference to FIG. 28, the outer housing 28 is assembled over theremainder of the unit 10 and bolted thereto at flange 380 and a flange400 of the housing 28, and at flange 402 of the assembly 237 and aflange 404 of the housing 28, preferably with a suitable gasket betweenthe flange connections to seal the connections.

With reference to FIG. 29, the assembly details associated with theupper electrodes 210 and the electrode sleeves 211 will be described inmore detail. Differential thermal expansion both in the radial directionrelative to the central axis 16 and in the longitudinal directionrelative to the central axis 16 present one challenge with respect tothe upper and lower electrodes 210 which must extend outside of thehousing 28 while preventing or allowing only a limited amount of leakageof the cathode flow. As illustrated in FIG. 29, the preferred embodimentof the unit 10 addresses this problem by providing slip rings that fitin two piece retainer structures. More specifically, a slip ring 410having a central bore 412 is assembled to the electrode 210 with a closefit between the exterior of the electrode 210 and the bore 412 in orderto restrict or prevent leakage while allowing relative movement betweenthe slip ring 410 and the electrode 210 in the longitudinal direction.The outer perimeter 414 of the slip ring 410 is received in an annularslot 416 of a two piece retainer structure 418 that forms the upper partof the electrode sleeve 211. The outer perimeter has a tight fit in theslot 416 so as to prevent or restrict leakage while allowing forrelative movement between the ring 410 and the retainer 418 in theradial direction, which in turn allows relative radial movement betweenthe electrode 210 and the housing 28. Together, the slip ring 410 andthe retainer 418 form a seal/slip ring assembly 418. Similar seal/slipring assemblies 422, 424 and 426 are provided for the interface betweenthe electrode sleeve 211 and the housing 28, the cover plate 368, andthe heat shield 358, respectively. Similar seal slip ring assemblies 428are shown in FIG. 5 for use with eight lower electrodes 210.

It should be appreciated that while the integrated unit 10 has beenshown to include the cathode recuperator 20, the anode recuperator 22,the reformer 24, and the anode exhaust cooler 26, in some applicationsit may be desirable to eliminate one or more of these components fromthe integrated unit 10. Conversely, it may be desirable in someapplications to add other components to the integrated unit 10. Forexample, with reference to FIG. 30, an alternate preferred embodiment ofthe unit 10 is shown and differs from the previously describedembodiment primarily in that a steam generator (water/combined exhaustheat exchanger) 440 has been added in order to utilize waste heat fromthe combined exhaust 62 to produce steam during startup. In this regard,a water flow 442 is provided to a water inlet port 444 of the heatexchanger 440, and a steam outlet port 446 directs a steam flow 448 tobe mixed with the anode feed 80 for delivery to the anode feed inletport 30. With reference to FIG. 31, the heat exchanger 440 includes acathode exhaust fin 450; an annular housing 452 having acircumferentially extending, three pass water flow path 454 formed in anexterior side thereof; and a water passage seal ring 456 that is bonded,such as by brazing, to the exterior of the housing 452 surrounding thewater flow path 454 so as to seal the same as best seen in FIG. 32. Thewater flow path 454 includes a first circumferentially extending pass458 that extends around almost the entire circumference of the housing452 to direct the water flow, shown by arrows 459, from the inlet 444 toa second circumferentially extending pass 460 of the flow path 454 whichextends almost around the entire circumference of the housing 452 todirect the water flow 459 to a third circumferentially extending pass462 of the flow path 454, which extends around almost the entirecircumference of the housing 452 to deliver the water flow 456, nowsteam, to the outlet 446. It can be seen that each of the passes 458,460 and 462 are formed so that they have a progressively larger flowarea from pass to pass so as to accommodate the increased volume as thewater changes from the liquid phase to the vapor phase. Preferably, thefin 450 is bonded, such as by brazing, to the interior surface of thehousing 452 to increase the transfer of heat from the exhaust flow 62 tothe water flow 459. While a preferred form has been disclosed herein forthe steam generator 440, it should be understood that other forms andconfigurations may be desirable, depending upon the requirements andparameters of each specific application.

The unit 10 of FIG. 30 also differs from the previously described unit10 shown in FIGS. 1-29 in that each stack 14 includes two additionalanode feed/return assemblies 94 and three additional sets of thecollector plates 200 that are not associated with any of the assemblies94. These modifications illustrate that in some applications more (orless) of the assemblies 94 may be required to achieve an optimumdistribution of the anode feed 80 to each of the stacks 14 and/or thatadditional assemblies 94 and collector plates 200 may be required inorder to optimize the electrical output of each of the stacks 14.

FIG. 33 is a schematic representation of the previously describedintegrated unit 10, including the preferred embodiment described inconnection with FIGS. 30-32, and showing the various flows through theintegrated unit 10 in relation to each of the major components of theintegrated unit 10. FIG. 33 also shows an optional air cooled anodecondenser 460 that is preferably used to cool the anode exhaust flow 39and condense water therefrom prior to the flow 39 entering the combustor38. FIG. 33 also shows a blower 462 for providing an air flow to thecombustor 38, a blower 464 for providing the cathode feed 44, and ablower 466 for pressurizing the anode recycle flow 42.

It should be appreciated that while several heat exchanger subassemblieshave been included in the integrated unit 10, many of the heatexchangers disclosed herein may prove desirable in other systems, oreven as stand alone assemblies.

It should also be appreciated that by arranging the fuel cell stacks 14into the array 12, the unit 10 can provide for a relatively compactstructure that minimizes the leakage of the cathode flow that cansometimes by associated with planar SOFC's. In this regard, it should benoted that the annular arrangement of the fuel cell stacks 14 incombination with the baffles 220, eliminates the need for specializedstructures to provide compression against the side walls of the fuelcell stacks such as is required in conventional planar SOFCconfigurations. It should also be appreciated that the integrated unit10 provides for an efficient utilization of the heat that is generatedwithin the unit 10.

Thus, the invention provides, among other things, a solid oxide fuelcell unit. Various features and advantages of the invention are setforth in the following claims.

1. A fuel cell unit comprising: a plurality of angularly spaced fuelcell stacks arranged to form a ring-shaped structure about a centralaxis, each of the fuel cell stacks having a stacking direction extendingparallel to the central axis, wherein each of the stacks has arectangular cross section; an annular cathode feed manifold surroundingthe fuel cell stacks to deliver a cathode feed flow thereto; a pluralityof baffles extending parallel to the central axis, each of the baffleslocated between an adjacent pair of the fuel cell stacks to direct acathode feed flow from the annular cathode feed manifold and radiallyinwardly through the adjacent pair; an annular cathode exhaust manifoldsurrounded by the fuel cell stacks to receive a cathode exhaust flowtherefrom; and a radial cathode exhaust flow passage located underneaththe ring-shaped structure and extending from the annular cathode exhaustmanifold in a generally symmetric 360 degree pattern to a locationradially outward of the annular cathode feed manifold.
 2. The fuel cellunit of claim 1, wherein each of the baffles has a wedge shaped crosssection that tapers in a radially inward direction relative to thecentral axis.
 3. The fuel cell unit of claim 1, further comprising: apair of pressure plates sandwiching the fuel cell stacks therebetween; aplurality of tie rods, each rod extending through a corresponding one ofthe baffles parallel to the central axis and engaged with the pressureplates; and at least one compression spring assembly cooperating withthe plurality of tie rods to compress the fuel cell stacks between thepressure plates while allowing for thermal growth differential of atleast one of the plurality of tie rods.
 4. The fuel cell unit of claim1, further comprising: a pair of pressure plates sandwiching the fuelcell stacks therebetween, one of the pressure plates comprising an anodefeed flow manifold assembly configured to direct an anode flow to andfrom each of the fuel cell stacks, and a plurality of tie rodscooperating with at least one compression spring assembly to compressthe fuel cell stacks between the pair of pressure plates.
 5. The fuelcell unit of claim 4, wherein the manifold assembly comprises: a firstcover plate having at least one anode feed inlet port to receive ananode feed flow from a remainder of the unit, a plurality of stack feedports to direct the anode feed flow to the fuel cell stacks and aplurality of stack exhaust ports to receive an anode exhaust flow fromthe fuel cell stacks; and at least one anode exhaust port to direct theanode exhaust flow to a remainder of the unit; at least one intermediateplate having slots and openings configured to direct the anode feed flowfrom the at least one anode feed inlet port to the plurality of stackfeed ports and to direct the anode exhaust flow from the plurality ofstack exhaust ports to the at least one anode exhaust port; and a secondcover plate, the at least one intermediate plate sandwiched between thefirst and second cover plates.
 6. A fuel cell unit comprising: anannular array of fuel cell stacks surrounding a central axis, with eachof the fuel cell stacks having a stacking direction extending parallelto the central axis, wherein each of the stacks has a rectangular crosssection; an annular cathode feed manifold having a first annular portionpositioned radially inward from the fuel cell stacks, a radial portionextending radially outward from the first annular portion in a generallysymmetric 360 degree pattern, and a second annular portion extendingfrom the radial portion generally parallel to the first annular portionand surrounding the annular array of fuel cell stacks to deliver acathode feed flow thereto; a plurality of baffles extending parallel tothe central axis, each of the baffles located between an adjacent pairof the fuel cell stacks to direct a cathode feed flow from the annularcathode feed manifold and radially inwardly through the adjacent pair;an annular cathode exhaust manifold surrounded by the annular array offuel cell stacks to receive a cathode exhaust flow therefrom; and aradial cathode exhaust flow passage located underneath the ring-shapedstructure and extending from the annular cathode exhaust manifold in agenerally symmetric 360 degree pattern to a location radially outward ofthe annular cathode feed manifold.
 7. The fuel cell unit of claim 6,wherein each of the baffles has a wedge shaped cross section that tapersin a radially inward direction relative to the central axis.
 8. The fuelcell unit of claim 6, further comprising: a pair of pressure platessandwiching the fuel cell stacks therebetween; a plurality of tie rods,each rod extending through a corresponding one of the baffles parallelto the central axis and engaged with the pressure plates; and at leastone compression spring assembly cooperating with the plurality of tierods to compress the fuel cell stacks between the pressure plates whileallowing for thermal growth differential of at least one of theplurality of tie rods.
 9. The fuel cell unit of claim 6, furthercomprising: a pair of pressure plates sandwiching the fuel cell stackstherebetween, one of the pressure plates comprising an anode feed flowmanifold assembly configured to direct an anode feed flow to and fromeach of the fuel cell stacks, and a plurality of tie rods cooperatingwith at least one compression spring assembly to compress the fuel cellstacks between the pair of pressure plates.
 10. The fuel cell unit ofclaim 9, wherein the manifold assembly comprises: a first cover platehaving at least one anode feed inlet port to receive the anode feed flowfrom a remainder of the unit, a plurality of stack feed ports to directanode feed to the fuel cell stacks and a plurality of stack exhaustports to receive an anode exhaust flow from the fuel cell stacks; and atleast one anode exhaust port to direct anode exhaust to a remainder ofthe unit; at least one intermediate plate having slots and openingsconfigured to direct the anode feed flow from the at least one anodefeed inlet port to the plurality of stack feed ports and to direct theanode exhaust flow from the plurality of stack exhaust ports to the atleast one anode exhaust port; and a second cover plate, the at least oneintermediate plate sandwiched between the first and second cover plates.11. The fuel cell unit of claim 6, further comprising at least oneradial cathode feed flow passage connected with an annular cathode feedflow passage surrounding the plurality of fuel cell stacks; and each ofthe fuel cell stacks includes a plurality of cathode feed flow paths,wherein the baffles cooperate with the at least one radial cathode feedflow passage and the annular feed flow passage to direct cathode feedflow to the cathode feed flow paths.
 12. The fuel cell unit of claim 11,further comprising an annular cathode exhaust flow passage in heatexchange relation with said annular cathode feed flow passage to definea cathode recuperator heat exchanger.
 13. The fuel cell unit of claim 3,wherein one of the pair of pressure plates at least partially definesthe radial cathode exhaust flow passage.
 14. The fuel cell unit of claim4, wherein the one of the pressure plates at least partially defines theradial cathode exhaust flow passage.
 15. The fuel cell unit of claim 8,wherein one of the pair of pressure plates at least partially definesthe radial cathode exhaust flow passage.
 16. The fuel cell unit of claim9, wherein the one of the pressure plates at least partially defines theradial cathode exhaust flow passage.
 17. The fuel cell unit of claim 12,wherein the annular cathode exhaust flow passage is operativelyconnected to the annular cathode exhaust manifold by way of the radialcathode exhaust flow passage.